Geophysical investigations can be a timesaving and cost-effective method for providing qualitative subsurface information for a site. They can be used for screening
large areas for potential buried wastes, for focusing resources for intrusive
investigation activities on the anomalous areas, and for identifying or
confirming the presence and extent of landfills and/or burn dumps. Buried solid waste and metals will (most likely) exhibit different bulk material properties than the surrounding native
soil. This will typically allow geophysical instruments to distinguish the waste form the soil.

The results obtained from a geophysical investigation are
subjective and rely on geologic interpretation. Geophysical techniques do not directly measure the parameter needed to solve the problem but instead measure contrasts in material
properties. For example, seismic methods measure velocities of seismic
waves through the subsurface material and electrical methods measure the
conductivity (or it’s inverse, resistivity) of a material.

The interpretation of geophysical contacts is based on
geologic assumptions: (1) earthen materials have distinct subsurface
boundaries, (2) a material is homogeneous (material properties are the same
throughout) and (3) the unit is isotropic (material properties are the same in
all directions). Since these conditions rarely occur in nature, and almost never occur in solid waste or burn ash, geophysical methods are most often used in conjunction with other intrusive
methods in order to more correctly assess the site.

Although geophysical interpretations are not always perfectly accurate, geophysical equipment is very precise. That is to say that the measurements obtained from non-intrusive geophysical techniques are very exact. The raw data is good data. The problem resides in the geophysical interpretation of the data, which are often educated estimations and/or calculated correlations and can lead to inaccuracies. However, when the appropriate geophysical technique is coupled with an intrusive investigation, large volumes of material can be explored accurately and cost-effectively.

Non-intrusive geophysical methods can be utilized as preliminary screening before performing intrusive investigations, they may be implemented as the primary investigative technique, they may be used in combination with intrusive investigation methods such as bore holes or test
pits, or they can be used in combination with other non-intrusive geophysical
methods. Understanding the specific strengths and weaknesses of each method will allow the investigator to decide how to best utilize geophysical investigation.

The following tables compare various methods and point out strengths and weaknesses:

Advantages and disadvantages of seismic methods

Applications of Geophysical Methods

A variety of non-intrusive investigative techniques can be
used to study environmental issues. The location and boundaries of buried wastes can be best approximated using
Ground Penetrating Radar,
Electromagnetics, and/or Resistivity. The
following are site-specific examples of successful use of geophysical
investigative techniques.

Site Investigation Reports

A table listing site investigations
completed by the CIWMB is available. The investigations are
listed alphabetically by site name and include: Solid Waste
Information System (SWIS) number, type of plan or report
produced, investigation type, types of operations, and whether
the site is located in a rural or urban setting. The table
identifies site investigations during which geophysical survey
operations were utilized.

General Links

Seismic geophysical methods use a seismic source and
receivers to “see” the subsurface using compressional waves. The velocity of the seismic waves are
recorded by the receivers, called geophones
(Spring-mounted electric coils moving within a magnetic field, which generate
electric currents in response to ground motion.), and
correlated to the material properties of the subsurface. Compressional
(P-waves) waves are generated by a hammer and propagate down into the
earth. Geophones “listen” for the waves
to return to the earth’s surface. Careful analysis can tell us whether it is a direct surface wave, one reflected
from a subsurface geologic interface, or a wave refracted
along the top of a geologic interface.

Seismic Refraction

Seismic refraction measures the seismic velocity of the
subsurface material, which is related to density and elastic properties and
therefore can be correlated to a material type. It is more commonly used for shallow subsurface investigations
than seismic reflection.

More information on seismic refraction

Seismic Reflection

Seismic reflection measures the seismic velocity of the
subsurface material, which is related to density and elastic properties and
therefore can be correlated to a material type. It is different from seismic refraction in that it records the
reflected seismic waves. Seismic
reflection is commonly used in oil exploration and for deep subsurface
exploration.

More information on seismic reflection

Geological materials have different electrical properties. The variations in these properties are useful geophysical
parameters for characterizing earthen materials. Subsurface variations in
electrical conductivity (or its inverse, resistivity) typically correlate with
variations in water content, fluid conductivity, porosity, permeability, and
the presence of metal. These variations may be used to locate subsurface
features whose electrical properties contrast with the surrounding earth. For example, decaying solid waste and metal
have a higher electrical conductivity than most soil and therefore produce
anomalous readings in measured conductivity readings.

The methods used to measure the properties of geologic
materials can be divided into two types: methods using applied currents and
those using naturally occurring currents. Those methods that used applied
currents include electrical resistivity, induced polarization, and
electromagnetic surveying. Methods that use naturally occurring current flow
include telluric surveying, magnetotelluric surveying, and the self-potential
method.

Two of the more commercially utilized techniques are the resistivity method and the electromagnetic (EM) method.
Resistivity can provide better vertical resolution and is generally less
sensitive to interfering noise such as fences, buildings and overhead power
lines. EM requires no direct contact with the ground surface, so the data can
be acquired more quickly than with resistivity. For more specific information on
all the previously mentioned methods, visit the
Southwest
Geophysics website.

Gravity is defined as the force of attraction between two
masses. The most commonly understood
gravitational force is between the sun and the earth. However, lateral density changes in the earth’s subsurface cause
a change in the force of gravity at the surface of the earth. A subsurface body of a different density
from its surroundings will attract a mass on the surface to a greater or lesser
extent than the surrounding earth. By
analyzing the change in gravitational attraction along the surface of the earth
these subsurface anomalies can be detected.

Geophysicists can use gravity measurements to help them
understand the internal structure of the earth while an environmental scientist
can use gravity measurements to locate underground gravitational anomalies. A
gravimeter is an instrument designed to measure spatial variations in gravitational
acceleration. There are various types of gravimeters
in existence today. The most
common gravimeter used in surveys is based on a simple mass-spring
system.

Gravity measurements alone are very difficult to analyze,
there are many solutions and interpretations to the observed measurements.
Gravity surveys and measurements are commonly used in conjunction with other
studies to confirm theories drawn by geologists. Mathematical corrections to measured anomalies are often
necessary and should be applied with the discretion of a geologist or
geophysicist.

More information on gravity and gravitational surveys

Defining Landfill Geometry Using a Gravimetric Survey

Roberts et al.* performed a gravity survey of a municipal
solid waste landfill in order to investigate the effectiveness of the method in
establishing the lateral boundaries and the vertical extent of the landfill.
The survey consisted of approximately 200 gravity stations spaced at 5-meter to
10-meter intervals. Bouguer and terrain corrections were made to the raw data. The information gathered from the gravity survey was compared
to data from boreholes and pre-landfill and post-landfill topographic
maps. The results of the survey closely
correlated with the topographic maps and borehole data.

To illustrate the importance of obtaining independent
geologic information about the site prior to performing a gravity survey,
Roberts stated “the gravity method depends on precise gravity and surface
elevation measurements, careful computations, and constraining information
obtained from collateral geologic and geophysical studies for the
interpretations to be valid.” (Roberts et al p. 259).

Whether on the surface or below, iron objects or minerals
cause local distortions or anomalies in the earth’s magnetic field. Magnetometers measure these variations in the magnetic field. Magnetometers were originally designed for
mineral exploration, but are now used in the environmental field for locating
buried steel drums, tanks, pipes, and iron debris in trenches and landfills. A
magnetometer detects local buried iron objects because the object causes the
(locally) uniform magnetic field of the Earth to strengthen or weaken depending
on the size, orientation, and magnetic characteristics of the object.

The magnetometer can only sense ferrous materials such as
iron and steel. Other metals like
copper, tin, aluminum, and brass are not ferromagnetic and cannot be located
with a magnetometer (but may be found with a metal detector). An object with weaker magnetic
characteristics may be detected at a maximum depth of about 5-10 feet. On the other hand, large masses of drums may
be detected easily to depths of 10-40 feet.

Example of a magnetic survey image

Select the image to view a larger version.

More information on magnetic surveys

Ground penetrating radar is a geophysical method that
generates a continuous profile of the subsurface. GPR’s radiate a very short burst of radio-frequency energy into
the ground to detect discontinuities. A transmitter antenna generates high
frequency radio waves that propagate into the earth in a broad beam. An echo from various subsurface interfaces
is reflected back to the observer from a remote target. The strength of the
echo is dependent on the absorption of the signal on the path to and from the
target, the size and shape of the target, and the degree of discontinuity at
the reflecting boundary. GPR’s detect a boundary between rock and air (a cave
or cavity) or between one type of soil and another (for example undisturbed
soil to waste). The dielectric properties of the subsurface materials correlate
with many of the mechanical and geologic parameters of these materials. The performance GPR is limited by
attenuation. The signals in moist soils, especially soils having high clay
content, can be significantly attenuated. GPR is most useful in detecting changes in the geometry of subsurface
interfaces.